Precipitated or dissolved alkali metal sulphates are obtained in many diverse chemical processing operations, such as in the production of chlorine dioxide and rayon, flue gas scrubbing and pickling of metals. In some cases, the sulphate is a resource even though the value can be rather limited. Thus, sulphate obtained from the manufacture of chlorine dioxide can be used for tall oil splitting and as a make-up chemical in kraft mills or as a filler in detergents. However, the amount of sulphate used in these areas has decreased steadily due to changing processing conditions. Disposal of the sulphate into the water body surrounding the plant, means an environmental problem. Furthermore, this means increased production costs, arising from the chemicals needed for neutralization prior to discharge. Also, this means a lost resource since the sulphate usually has to be replaced with purchased chemicals. An efficient process to recover alkali metal sulphates in usable form and concentration has, therefore, been desirable for a considerable period of time.
Electrodialytic water splitting is a well known technology aimed at the problem with efficient recovery of sulphates. In this process, an aqueous solution containing sulphate of various origins is brought to an electrolyzer equipped with at least one diaphragm or membrane. By applying a direct electric current, the sulphate and water are split into ions, which react to produce sulphuric acid in the anolyte and a hydroxide in the catholyte.
In electrodialytic water splitting, the sulphate electrolyte used is normally purified. This has been considered especially important with membrane cells, which are much more sensitive to impurities than diaphragms. Thus, in the absence of substantial purification measures under alkaline conditions, magnesium and calcium hydroxide can precipitate in and on the membranes and on the electrodes. This will bring about increased operating voltage and reduced current yield. The purification commonly consists of precipitation and subsequent filtration followed by ion exchange. A requirement for this purification technique is the dissolution of the sulphate. This means that hitherto, the maximum concentration of sulphate in the anolyte feed has been limited by the solubility of the sulphate prior to electrolysis. The effect of this limitation has been a low concentration of sulphuric acid produced, i.e. normally in the order of 8-15 percent by weight.
According to EP 449071, alkali metal hydroxide and sulphuric acid are produced by electrodialytic water splitting of an aqueous solution containing dissolved sulphate. The three compartment membrane cell is equipped with special anion and cation exchange membranes, to reduce the sensitivity towards impurities and to allow for the production of concentrated sulphuric acid and hydroxide. For the same reasons, ammonium or amines are added to the sulphate solution fed to the intermediate salt compartment.
According to U.S. Pat. No. 4,129,484, chlorine dioxide is produced in a process by reducing chlorate with e.g. sulphur dioxide. The residual solution containing sulphate and unreacted sulphuric acid, is brought to an electrochemical membrane cell having two or three compartments where the sulphate is split. According to one embodiment, the cell is divided into two compartments by means of a cation exchange membrane. The residual solution is introduced into the anode compartment and the solution withdrawn from the anode compartment enriched in acid. This acid can be brought back to the chlorine dioxide generator, for further acidification in the reduction of chlorate.
Although several electrodialytic water splitting processes are known for the production of sulphuric acid and alkali metal hydroxide from alkali metal sulphate, the concentration of the products and the energy efficiency have hitherto been limited. Therefore, electrodialytic water splitting has not yet been widely recognized as an economic alternative when dealing with waste alkali metal sulphates. It is the aim of this invention to provide an efficient process with few steps, by which highly concentrated and pure products can be produced.